The CIA and MI6 have regularly given large cash payments to Hamid Karzai's office with the aim of maintaining access to the Afghan leader and his top allies and officials, but the attempt to buy influence has largely failed and may have backfired, former diplomats and policy analysts say.

The Guardian understands that the payments by British intelligence were on a smaller scale than the CIA's handouts, reported in the New York Times to have been in the tens of millions, and much of the British money has gone towards attempts to finance peace initiatives, which have so far proved abortive.

That failure has raised questions among some British officials over whether eagerness to promote a political settlement may have been exploited by Afghan officials and self-styled intermediaries for the Taliban.

Responding to the allegations while on a visit to Helsinki on Monday, Karzai said his national security council (NSC) had received support from the US government for the past 10 years, and the amounts involved were "not big" and were used for a variety of purposes including helping those wounded in the conflict. "It's multi-purpose assistance," he said, without commenting on the allegations that the money was fuelling corruption.

Yama Torabi, the director of Integrity Watch Afghanistan said that the presidency's low-key response to the reports had "outraged people".

"As a result, we don't know what was the amount of money that was given, what it was used for and if there was any corruption involved. Money when it is unchecked can be abused and this looks like one. In addition, it can be potentially used to corrupt politicians and political circles, but there is no way to know this unless there is a serious investigation into it," Torabi told The Guardian.

Kabul sources told the Guardian that the key official involved in distributing the payments within the NSC was Ibrahim Spinzada, a close confidant of the president known as Engineer Ibrahim. There is, however, no evidence that Spinzada personally gained from the cash payments or that in distributing them among the president's allies and sometimes his foes he was breaking Afghan law.

Officials say the payments, referred to in a New York Times report as "ghost money", helped prop up warlords and corrupt officials, deepening Afghan popular mistrust of the Kabul government and its foreign backers, and thereby helped drive the insurgency.

The CIA money has sometimes caused divisions between the various branches of US government represented in Kabul, according to diplomats stationed in Kabul, particularly when it helped give the CIA chief of station in Kabul direct access to Karzai without the US ambassador's knowledge or approval.

One former Afghan budgetary official told the Guardian: "On paper there was very little money that went to the National Directorate of Security [NDS, the Afghan intelligence service], but we knew they were taken care of separately by the CIA.

"The thing about US money is a lot of it goes outside the budget, directly through individuals and companies, and that opens the way for corruption."

Khalil Roman, who served as Karzai's deputy chief of staff from 2002 until 2005, told the New York Times: "We called it 'ghost money'. It came in secret, and it left in secret."

One American official told the newspaper: "The biggest source of corruption in Afghanistan was the United States."

Sources said the MI6 aid was on a smaller scale, and much of it was focused on trying to promote meetings between Karzai's government and Taliban intermediaries, as was embarrassingly the case in 2010 when MI6 discovered a would-be Taliban leader in talks with Karzai was an impostor from the Pakistani city of Quetta.

The British payments have also been designed to bolster UK influence in Kabul, in what a source described as "an auction with each country trying to outbid the other" in the course of an often fraught relationship with the Karzai government.

Vali Nasr, a former US government adviser on Afghanistan, said: "Karzai has been lashing out against American officials and generals, so if indeed there has been funding by the CIA, you have to ask to what effect has that money been paid. It hasn't clearly brought the sort of influence it was meant to."

Nasr, now dean of the Johns Hopkins school of advanced international studies and author of a new book criticising US policy in Afghanistan, The Dispensable Nation, said: "If the terms of such payments are not clear, the question is how well do they tag with US policy … The CIA has a narrow, counter-terrorism purview that involved working with warlords, but that is quite a different agenda, on how we conduct the war or how we build a government."

The CIA has also been heavily criticised for conducting drone attacks against suspected militants over the border in Pakistan and for calling in air strikes inside Afghanistan while on joint operations with NDS units, leading to civilian casualties. A report on Monday by the Afghanistan Analysts Network, a thinktank in Kabul, said the latest such NDS-CIA operation, in Kunar province on 13 April, killed 17 civilians.

Kate Clark, one of the network's analysts, said: "It is one thing to conduct covert operations in a hostile country. I'm flabbergasted that the CIA is running these kind of covert operations in a friendly country. It runs counter to accountability, democracy and the rule of law, and is damaging what the US is trying to do.

"The CIA puts certain things as a priority – whether someone is against al-Qaida, for example – and damn the rest."